1. Field of the Invention
The present invention relates to a vari-focal photographic lens system and, more particularly, to a vari-focal photographic lens system of rear lens exchange type of which the focal length is variable and which is to be used with a camera with a lens shutter (hereinafter referred to as a popular camera).
2. Description of the Prior Art
As one of known methods for varying the focal length of a photographic lens system, there is a method to attach an auxiliary lens system in rear of a master lens system. This type of lens system is disclosed, for example, in Japanese published unexamined patent application No. 46224/82. However, in case of said type of lens system, the master lens only is also used for photographing without attaching the auxiliary lens system and, therefore, it is essential to favourably correct aberrations of the master lens system itself. This means that the master lens system itself and auxiliary lens system itself should be respectively arranged that aberrations thereof are favourably corrected independently of each other and, consequently, correction of aberrations is difficult. To solve the above-mentioned problem, it is necessary to arrange that each of the master lens system and auxiliary lens system has a complicated lens configuration and, therefore, the lens system as a whole including the master and auxiliary lens systems are not compact. Moreover, the number of lenses constituting the lens system as a whole becomes large, and this is not desirable because that cost of production becomes high.
As another method for varying the focal length of a photographic lens system, it is known to exchange some of lens components constituting the lens system. This type of lens system is disclosed, for example, in Japanese published unexamined patent application No. 132305/81. The lens system disclosed in said patent application consists of three lens components, i.e., a positive, negative and positive lens components, and is arranged to vary the focal length by exchanging the negative lens component which is arranged as a second lens component. In case of said method, the second negative lens component is to be inserted to and taken out of a small space between the first and third lens components and, therefore, it is difficult to secure a space for providing a mechanism for attaching and detaching the second lens component. Moreover, in case of a lens system consisting of three lens components, i.e., a positive, negative and positive lens components, the eccentricity of the second negative lens component generally causes a large decrease in the quality of the image. Therefore, when said method to exchange the second negative lens component is adopted, it is difficult to ensure a favourable quality of image.